The present invention relates to a sheath-core composite conductive fiber.
Composite fibers produced by coating a conductive component containing conductive particles with a non-conductive component have conventionally been used as conductive fibers.
In Europe and America, a method of measuring a resistance value while contacting an electrode at two positions on the surface of a textile product (hereinafter referred to a surface resistance measuring method) has recently been employed as a means for evaluating the conductivity without breaking the textile product. This method has the problem that the measured apparent conductivity is low, namely, the measured resistance value becomes higher in case of a conductive yarn wherein a conductive yarn is mixed with a textile product not having a surface conductive layer because a conductive component is not contacted with an electrode.
It is easy for us to get a suggestion that the surface layer is made of a conductive component in order to solve such a drawback, and various suggestions have been made. For example, a method of coating the surface with a metal such as titanium oxide or cuprous iodide has been suggested. According to such a method, the resulting product has insufficient washing durability and exhibits high conductivity at an initial stage, but the metal is peeled off during washing, thereby to lower conductive performances. Therefore, the method is not suited for use in dust-free clothes which indispensably require washing.
Although a sheath-core composite fiber comprising a sheath composed of a conductive layer containing carbon black incorporated therein has been suggested in Japanese Examined Patent Publication No. 57-25647, it was not a product suited for practical use because sheath-core formation of the sheath-core composite fiber is not easily performed. Since the presence of carbon black drastically lowers the spinnability of a thermoplastic resin, a core portion and a sheath portion of a composite component differ in thermal fluidity, and thus the spinnability drastically becomes worse. Furthermore, there was the problem that the operability is also lowered in post processes such as drawing process and weaving/knitting process because the sheath-core composite shape partially becomes un-uniform due to the same reason.
An object of the present invention is to provide a sheath-core composite conductive fiber which is superior in conductivity in a surface resistance measuring method and durability of conductivity, and also which has good passableness in the spinning process and the post process.
The present inventors have studied with paying attention to the fact that the coherency and waviness of a conductive fiber are improved and the passableness in the post process is remarkably improved by controlling the center of an inscribed circle of a sheath component in a cross section of a sheath-core composite fiber obtained by a melt-spinning process, which comprises a sheath component made of a fiber-forming polymer containing conductive carbon black, within a specific range, thus completing the present invention.
A first invention of the present invention provides a sheath-core composite conductive fiber comprising a sheath component made of a fiber-forming polymer containing conductive carbon black, characterized in that, with respect to an inscribed circle of a core component and an inscribed circle of a sheath component in a cross section of the fiber, a radius (R) of the inscribed circle of the sheath component and a distance (r) between the centers of two inscribed circles satisfy the following relationship:
r/Rxe2x89xa60.03xe2x80x83xe2x80x83{circle around (1)}
In a preferred aspect of the first invention, the carbon black content of the sheath component is within a range from 10 to 50% by weight.
In a more preferred aspect, a core-sheath ratio is within a range from 20:1 to 1:2 in terms of an area ratio of the core component to the sheath component.
A second invention of the present invention provides a sheath-core composite conductive fiber comprising:
a core component made of a polyester containing ethylene terephthalate as a main component, and
a sheath component made of a mixture of a copolyester wherein ethylene terephthalate accounts for 10 to 90 mol % of constituent units thereof and carbon black.
In a preferred aspect of the second invention, the sheath component is a polyester prepared by copolymerizing isophthalic acid and/or orthophthalic acid and/or naphthalenedicarboxylic acid as the copolymer of the acid component.
In a more preferred aspect, a copolymerization ratio of isophthalic acid and/or orthophthalic acid and/or naphthalenedicarboxylic acid as the copolymerization component is within a range from 10 to 50 mol %.
In a more preferred aspect, the carbon black content of the sheath component is within a range from 10 to 50% by weight.
In a more preferred aspect, a core-sheath ratio is within a range from 20:1 to 1:2 in terms of an area ratio of the core component to the sheath component.